Appendix 2.4.0. RATIONAL METHOD  

The Rational Method is based on the direct relationship between rainfall and runoff, and is expressed by the following equation:

Q _p = CiA (Eq. 2-1)

Where:

Q _p  is defined as the peak runoff in cubic feet per second. Actually, Qp is in units of acre-inches per hour. Since this rate of acre-in/hr differs from cubic feet per second by less than one (1) percent (1 acre-in/hr = 1.008 cfs), the more common units of cfs are used.

C is the composite coefficient of runoff representing the ratio of peak runoff rate "Qp" to average rainfall intensity rate "i" for the soil types and land uses characteristic of the contributing drainage area.

I is the average intensity of rainfall in inches per hour for a period of time equal to the time of concentration (tc) for the drainage area to the design point under consideration.

A is the area in acres contributing runoff to the point of design.

The following basic assumptions are associated with the Rational Method:

A.

The storm duration is equal to the time of concentration.

B.

The computed peak rate of runoff at the design point is a function of the average rainfall rate over a duration equal to the time of concentration at that point.

C.

The return period or frequency of the computed peak flow is the same as that for the design storm.

D.

The necessary basin characteristics can be identified and the runoff coefficient does not vary during a storm.

E.

Rainfall intensity is constant during the storm duration and spatially uniform for the area under analysis.

F.

The maximum rate of discharge at the point of design will occur when the entire area above the point of design is contributing runoff.